grp - exhaust - fire risk

[abbott013]

I have got to get a new high rise manifold made but am limited by height, what i would like to know is how close can i run a exhaust manifold elbow to the under side of my grp cockpit sole with out risk of fire, the manifold will be wrapped in heat resistant exhaust tape to finnish.

 

[Talulah]

I have got to get a new high rise manifold made but am limited by height, what i would like to know is how close can i run a exhaust manifold elbow to the under side of my grp cockpit sole with out risk of fire, the manifold will be wrapped in heat resistant exhaust tape to finnish.

To give a comparison my Eberspacher exhaust ducting runs through a hole in a fibreglass wall. There is some scorching around the hole but it will never be hot enough to worry about it.
Our high rise is wrapped in Fibreglass bandage and the bandage has no signs of heat damage.
I would want 50mm clearance above the high rise. This will make wrapping, removal, engine movement etc all perfectly easy and safe.
However, you don't want the high rise itself too high as this has a lever arm moment on the engine block itself. You would not want to break part of that off.
In other words the High Riser extension can be too high as well as too low.

 

[abbott013]

not sure how critical this is ? 25mm of gap between exhaust and grp. more views would be welcome

 

[ianabc]

and the reason for plastic exhaust is...

And the reason for a plastic exhaust is?

 

[abbott013]

And the reason for a plastic exhaust is?

its 316 stainless

 

[Boathook]

Leave a bigger gap as possible but don't allow the 'wrapping' to touch the GRP. A 25mm airgap should be ok but it does depend on how hot the exhaust is likely to get. I have seen heat transfer through 'abestos' board set ply on fire when there was a small airgap.

 

[lw395]

Some thin aluminium sheet between the exhaust and the GRP will spread the heat as well as reflecting it away. I would mount the ali on spacers, say 5mm? away from the grp.
But if there is no airflow, the heat will still build up.
At least with a bit of metal there you can monitor the heat after running the motor for a while.
I don't have a figure for a safe distance of exhaust to GRP.
You might find people who deal with rally or race cars could advise?

 

[abbott013]

thanks for that, thought i might have got a bit better responce on here. time to start googling

 

[lw395]

I think the lack of response is because it's hard to be exact about it.
Both my boats and all the ones I've fiddled with the engines had the mixing elbow right next to the manifold, so no big length of hot exhaust. I think, on the whole, that's the way I should want it. But it's not the only way.
I do know that heat shields a bit like I described make a huge difference on motorbike exhausts in some circumstances.
I think the wrapping will be important, you do not want the exhaust heating up the engine box much IMHO.
Maybe power boat people would have more relevant experience?
Sorry not to be more help.